In the past decade, there has been enormous development in the area of wireless communication systems. Particularly, wireless local area network (WLAN) technology has emerged as one of the prevailing wireless technologies throughout the world. Also, in the fourth-generation, the WLAN technology may play an important role in the wireless and mobile communication systems.
Typically, WLAN devices may operate in accordance with one or more protocol standards including, but not limited to, IEEE 802.11. Moreover, the protocol standards for the WLAN devices are upgraded based on certain factors, such as the data rate and throughput requirements, in the communication system. For example, the IEEE 802.11 specification has evolved to IEEE 802.11 standard, which later evolved to IEEE 802.11b standard. Further, the 802.11b standard has evolved to IEEE 802.11g standard and then to IEEE 802.11n standard.
In addition, the WLAN devices may include legacy devices, high throughput (HT) devices, and very high throughput (VHT) devices. The legacy devices are compliant to IEEE 802.11abg standards. Also, the legacy devices may provide a maximum data rate of 54 Mbps. On the other hand, the high throughput (HT) devices are compliant to IEEE 802.11n standard. With the development of MIMO-OFDM technology, the HT devices may provide a maximum data rate of 600 Mbps. In a similar manner, the VHT devices are compliant to IEEE 802.11ac standard. Moreover, there has been development in a very high throughput physical layer and a medium access controller (MAC) layer of the VHT devices to support a data rate of more than 1 Gbps.
Typically, a device that is complaint to IEEE 802.11n standard should be backward compatible to the legacy devices supporting a legacy frame format in addition to HT mixed and green-field formats. Similarly, a device that is complaint to IEEE 802.11ac standard should be backward compatible to the legacy devices and HT devices supporting legacy and HT frame formats in addition to its own VHT mixed format. To achieve this compatibility, a device at a receiver end should be capable of detecting the format of the frame for successful decoding of the frame/packet. For proper detection of the format, the receiver device should first have accurate time synchronization that suits for all frame formats in VHT wireless LAN.
Since OFDM modulation is employed in VHT wireless LAN, the receiver device may be very sensitive to time and frequency synchronization. Without proper time synchronization, the orthogonality of the subcarriers will be lost and there will be inter symbol interference (ISI) and inter carrier interference (ICI) in the demodulated data. This ISI and ICI may further cause many bits errors in the decoded data. Moreover, with multiple antennas at the transmitter end and the receiver end, the synchronization problem will be much more complicated. Thus, for all these reasons, there is a need for proper time synchronization in the wireless communication systems.
In a conventional system, the time synchronization is based on the correlation between a received preamble and a transmitted preamble of a frame/signal. However in the VHT wireless LAN, since the same preamble is transmitted from the different antennas with different cyclic shifts and there are different frame formats to be supported, the computed correlation between the received preamble and the transmitted preamble may be spread by unknown amount at the receiver. Because of this spread, the time synchronization may not be accurate and there will be ISI and ICI in the demodulated data, which cause many bit errors in the decoded data.
It is therefore desirable to develop a robust and adaptive time synchronization technique that suits for all frame formats in VHT wireless LAN and works well for signals with variable cyclic shifts applied at the transmitter. Also, the synchronization technique should be capable of synchronizing the receiver for all frame formats even under variable multipath channel delay spread and fading conditions.